The invention is generally in the field of laser-induced breakdown spectroscopy (LIBS) and more particularly to a method and a system for conducting LIBS analysis of material moving on a conveyor system.
For various applications, methods are needed for determining the material constitution of a sample. A known method is laser-induced breakdown spectroscopy (LIBS), which involves focusing a laser beam onto a surface of the sample with a high enough power density to transform a small part of the sample material into a state of plasma. Optical emissions from the plasma plume are collected with light collection optics, and the spectral distribution (i.e. intensity as a function of wavelength) of the collected optical emissions is analyzed in a wavelength-sensitive detector such as a spectrometer that produces information in electronic form describing the spectral distribution. Since atomic and molecular constituents of sample materials have characteristic optical emission spectra, the information produced by the spectrometer reveals the constituents of that part of the sample onto which the laser beam was focused or directed.
The sample may in principle be solid, liquid or gaseous. In the case of a gaseous sample the concept of a “surface” of the sample does not exist, but the laser beam is just focused into the gaseous sample. Drawbacks of known LIBS measurement devices include their bulky structure and limited applicability to field use. In certain applications, LIBS measurement systems are able to characterize various samples in terms of elemental concentrations. In other applications, the type of material, such as a particular alloy composition can be identified with or without the determination of exact elemental concentrations. A primary advantage of the LIBS technology over XRF (x-ray fluorescence) is its ability to determine the elemental concentration of light elements such as Li, B, Be, C, Al, Na and Mg.
In U.S. Pat. No. 6,795,179 to Kumar, there is disclosed a metal particle sorting system that includes a conveyor for conveying randomly shaped scrap metal pieces in a random orientation, an image detector for electronically recording the image of a predefined viewing area through which the scrap pieces are conveyed by a traditional flat conveyor system, a position detector for detecting movement of the conveyor belt, a laser system configured to provide a laser beam including a stream of a plurality of laser pulses within a selected time interval, and at least one laser scanner assembly including a positionable beam deflector to direct the laser pulses at a selected piece at any location in a selected target region on the conveyor and a focusing element mounted downstream of the source of the laser pulses from the beam deflector to focus the beam and provide uniform laser power density along a plane. The system further includes a light collector for collecting light from plasma produced on the pieces as they are irradiated by the laser pulses, a light distribution and spectral analyzer system for isolating and measuring at least one selected band from the collected light, a separator to divert pieces to different bins based on discriminator signals, and control logic for continuously acquiring an image of the selected viewing area of the conveyor, processing the image to identify and locate the scrap pieces as they pass through the viewing area, monitoring the laser system to determine when the next laser pulses will be available, selecting a downstream location on the conveyor at which the next available stream of pulses of radiation may be directed at an identified piece, operating the scanner assemblies as required to direct the pulses to the selected target location, analyzing spectral data collected from the plasma, generating a discriminator signal based at least in part upon the spectral data analysis, and selectably activating the separator as a function of the discriminator signal to sort the analyzed pieces. Although apparently effective, this system is equipment- and investment-intensive as several scanner assemblies for generating plasma samples are required as well as control logic for continuously acquiring and processing images of the selected viewing area of the conveyor among other components.
In U.S. Pat. No. 6,753,957 to Graft et al., a system and a method is disclosed for mineral sorting and detecting, including remote sensing, and more particularly, for real-time detection and content evaluation of minerals or trace concentrations of elements in materials as they are conveyed on a moving belt. The Graft et al. invention employs a laser-induced breakdown spectroscopy (LIBS) system wherein intensity ratios of the emission lines characteristic of specific elements or minerals enable detection of the same while on a moving belt. Because associated minerals have different chemical compositions, namely, major or minor elements, the relative intensities, defined by their characteristic spectral lines, enable all phases to be consistently identified and assessed within a short time that is consistent with both LIBS and the moving belt system.